Linux 6.8 compat: make test functions static

The kernel is now being compiled with -Wmissing-prototypes. Most of our
test stub functions had no prototype, and failed to compile. Since they
don't need to be visible anywhere else, just make them all static.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #15805